CF patients with impaired glucose tolerance due to defective secretion are not routinely treated with exogenous insulin since fasting glucose and hemoglobin A1c levels are normal. This practice ignores the potentially detrimental effects of insulin deficiency on protein metabolism. We hypothesize that chronic treatment with either insulin or an oral hypoglycemic agent will lead to improved protein anabolism in insulin-deficient, non-diabetic CF adults. A multicenter, prospective, placebo-controlled, double-blinded intervention trial will be undertaken to treat glucose intolerant adult CF patients for six months wiht one of three agents: insulin, glipizide, troglitazone or placebo. We will evaluate the effects of these agents on protein synethesis and breakdown, pulmonary function, and body composition.